This invention relates to a stage-drive control apparatus and method for simultaneously driving two stages and, more particularly to a scan-projection type exposure apparatus and method which sequentially transfer-exposes a mask pattern on wafers in a slit-scan exposure method in a photolithographic process for manufacturing semiconductor devices, liquid-crystal display devices and the like.
Upon manufacturing semiconductor devices or liquid-crystal display devices by the photolithography process, a projection-exposure apparatus is used to transfer a pattern formed on a reticle onto a wafer to which sensitizer material is applied.
Conventionally, as a projection-exposure apparatus, the so-called step-and-repeat type reduced projection-exposure apparatus operates in many production sites. This apparatus sequentially moves respective areas to be exposed (shot areas) of wafers within an exposure field of an optical projection-exposure system, thus sequentially exposing a reticle pattern image on the respective shot areas. However, the size of semiconductor chips has increased in recent years, and the projection-exposure apparatuses are required to have a large exposure area to transfer-expose larger reticle patterns on wafers. At the same time, in response to microminiaturization of semiconductor patterns, the resolution of the reticle must be improved. However, it is technically difficult to design and manufacture a projection-exposure apparatus that has an improved resolution and a large exposure area.
To solve this problem, the so-called scan-projection type exposure apparatus has been developed. This apparatus scans (moves) a reticle with respect to a slit illumination area. The reticle pattern is sequentially exposed on wafers by scanning (moving) the wafers in synchronization with the reticle with respect to an exposure area in conjugation with the illumination area.
FIG. 5 is a cross-sectional view showing the conventional scan-projection type exposure apparatus. In FIG. 5, reference numeral 1 denotes an optical illumination system; 2, exposure light emitted from the optical illumination system 1; 3, a reticle held on a reticle stage 4; 5, an optical projection system; 6, a wafer to which a photoresist is applied; and 7, a wafer stage. The reticle stage 4 and the wafer stage 7 move in the y-axial direction opposite to each other, respectively, at a speed V.sub.r and a speed V.sub.w.
A master-slave synchronized control system is known as a control principle for moving the two positioning objects at a fixed speed while maintaining a synchronized relation between the objects. This principle will be briefly described with reference to FIG. 6, which is a block diagram showing the master-slave synchronized control system. In FIG. 6, the relation among values x.sub.M, d.sub.M, x.sub.S, d.sub.S and r.sub.M is represented as follows. Note that d.sub.M '=0 holds for the sake of simplification. ##EQU1## r.sub.M : target value to master control system r.sub.S : target value to slave control system
e.sub.M : positional error of master control system PA1 e.sub.S : positional error of slave control system PA1 G.sub.MC (s): compensator of master control system PA1 G.sub.SC (s): compensator of slave control system PA1 G.sub.M (s): controlled object of master control system PA1 G.sub.S (s): controlled object of slave control system PA1 d.sub.M : force-based disturbance to master control system PA1 d.sub.M ': displacement-based disturbance to master control system PA1 d.sub.S : force-based disturbance to slave control system PA1 X.sub.M : displacement output of master control system PA1 X.sub.S : displacement output of slave control system PA1 .beta.: reduction ratio (e.g., 1/4)
In FIG. 6, the dashed-lined block represents a synchronization error e.sub.syc as an evaluation index defined by: EQU e.sub.syc =X.sub.M -.beta.X.sub.S ( 3)
Accordingly, the following relation is obtained by substituting equations (1) and (2) into equation (3): ##EQU2##
FIG. 7 shows the meaning of equation (2), i.e., an equivalence block diagram of the master-slave synchronized control system. In FIG. 7, the output X.sub.M of the master control system is multiplied by 1/.beta. as the target input to the slave control system. The disturbance d.sub.M inputted into the master control system is also transmitted to the slave control system, and as a result, controls the synchronization error e.sub.syc. On the other hand, the disturbance d.sub.S inputted into the slave control system is used as a disturbance control capability of a closed loop (loop comprising G.sub.SC (s) and G.sub.S (S)) of the slave control system.
In this master-slave synchronized control system, when the position signal X.sub.M of the master control system fluctuates due to the disturbance d.sub.M applied to the controlled object G.sub.M (s) of the master control system, the positional-error signal e.sub.M of the master control system is inputted into the slave control system via a synchronization correction path 13, with a unit having a 1/.beta. gain, the slave control system swivels in correspondence with the swivel of the master control system, thus maintaining the synchronized relation between the master and slave control systems. Note that "maintaining the synchronized relation" means setting the synchronization error e.sub.syc, in the dotted-lined block in FIG. 6, to zero.
As is apparent from the above description, no synchronization correction is performed on the disturbance d.sub.S, which is applied to the controlled object G.sub.S (S) of the slave control system, because there is no synchronization correction loop which passes a signal from the slave control system to the master control system. As described above, the disturbance is suppressed only by the disturbance control capability of the slave control system.
Next, a case wherein the wafer stage 7 comprising a fine-motion stage 8 and a rough-motion stage 9 is integrally driven in a negative y-axial direction at a fixed speed will be described with reference to FIG. 8A.
The position signal is detected by irradiating a light beam 10 of a laser interferometer (not shown) by a reflection mirror 11, and the fixed-speed drive is performed based on the detected signal. However, it should be noted that the fine-motion stage 8 is driven by actuators 12M, 12R and 12L and the like on the rough-motion stage 9, and when the fine-motion stage 8 is driven while the rough-motion stage 9 is driven, the reflection mirror 11 for position detection is also moved, which causes a disturbance in the fixed-speed drive.
For example, when the actuator 12M is extended, on the other hand, the actuators 12R and 12L are contracted to tilt the fine-motion stage 8 as shown in FIG. 8B, the rough-motion stage 9, which is being driven at the fixed speed in the negative y-axial direction, instantly causes a disturbance in the scanning speed. In this case, the scanning speed increases. The disturbance is equivalent to the disturbance d.sub.M ' (see FIGS. 6 and 7) applied to the master control system, and the master-slave synchronized control system sets the synchronization error to zero by its synchronization correction operation.
Next, a case wherein the actuators 12M, 12R and 12L and the like are expanded by .DELTA.Z as shown in FIG. 8C will be described. Similar to FIG. 8B, upon expansion of the actuators, when the surface of the reflection mirror 11 for receiving the light beam 10 is tilted from an axis z, the fixed speed drive of the rough-motion stage 9 being driven in the negative y-axial direction is disturbed. Further, it is apparent that when the fine-motion stage 8 is rotated around the axis z by another actuator (not shown), the fixed-speed drive of the rough-motion stage 9 is influenced by the rotation movement of the fine-motion stage 8.
As described above, the z-axial directional translation and the rotations about the x-, y-, and z-axis of the fine-motion stage 8 disturb the fixed-speed drive of the rough-motion stage 9. However, the master-slave control system as shown in FIGS. 6 and 7 controls the synchronization error to zero.
However, the synchronization-error control by the above master-slave control system is relatively slow. Accordingly, to further improve the scan-exposure capability, the synchronization error caused by the disturbance in the fixed-speed drive of the rough-motion stage 9 due to the drive of the fine-motion stage 8, must be quickly suppressed.
Note that Japanese Patent Application Laid-Open Nos. 7-29801 and 3-282605 disclose well-known techniques related to the present invention. Japanese Patent Application Laid-Open No. 7-29801 discloses a hybrid reticle stage having a rough-motion stage and a fine-motion stage. The rough-motion stage, having a speed control system, follows a target scanning speed independently of a wafer stage, while the fine-motion stage, having a position control system, performs synchronization correction. That is, this construction is basically the master-slave synchronized control system wherein the wafer stage serves as a master of the position control system while the reticle stage serves as a slave of the position control system. Further, to speed the response of the reticle stage as the slave, this system feeds-forward the difference between the speed of the wafer stage and that of the reticle stage to the reticle fine-motion stage.
Japanese Patent Application Laid-Open No. 3-282605 discloses a master-slave synchronized control method with speed feedforward function.
However, these techniques lack any means to positively control the displacement-based disturbance.
As described above, in a system which uses a wafer stage and a reticle stage as a master and slave so as to fixed-speed drive the wafer stage and the reticle stage at a fixed speed while maintaining a synchronized relation between them, i.e., a so-called master-slave synchronized control system, the wafer stage receives displacement-based disturbance due to leveling and tilting of a fine-motion stage of the wafer stage. This disturbance can be controlled by a loop structure of the master-slave synchronized control system.
When force-based disturbance such as d.sub.M and d.sub.S are inputted, the response is mitigated by mechanical systems such as G.sub.M (s) and G.sub.S (s), and appear as displacement outputs. On the other hand, the displacement-based disturbance such as d.sub.M ' is not mitigated and directly appears in the displacement output, which seriously influences the synchronization control capability. Accordingly, the scan-projection type exposure apparatus is required to quickly suppress the disturbance in the synchronization between the wafer stage and the reticle stage caused by the displacement-based disturbance.